This invention relates to a SYSTEM FOR DISPOSING OF RADIOACTIVE WASTE, and more particularly to a method and apparatus for solidifying radioactive waste liquids into a hardened mass suitable for disposal by burying.
Conventionally, liquid radioactive wastes, such as the liquid waste materials from a nuclear reactor, are disposed of by burying them in the earth or at sea. To prevent contamination of the surrounding environment after burial, the liquid materials, often mixed with solid radioactive waste materials, are usually solidified in some manner so they will remain where buried and will not leak or shed the radioactive materials.
Various attempts have been made to obtain solidification of these materials in a commercially acceptable manner. However, a number of problems are encountered and none of the prior systems are completely satisfactory. Solid radioactive wastes have been fixed in glass, bitumen, asphalt and similar materials. Efforts have been made to solidify aqueous slurries and solutions with such materials as emulsified asphalt, polyester and polyethylene. The latter have not proved to be practicable and almost all of the encapsulation of liquid radioactive waste materials has been accomplished by mixing it with Portland cement and allowing the resulting concrete-like mass to harden.
Many hitherto unsolved problems arise from the use of Portland cement to solidify the radioactive waste liquids. In the first place, the physical attributes of a solidified mass of concrete are jeopardized when variables are introduced such as a high range of pH values, many types of salt concentrations, and difficult materials, such as resin beads, in which the smooth surface areas differ greatly from the rough edges of the sand normally used. Concrete is a mixture of Portland cement, sand, aggregate (normally washed gravel), and water. The lack of the aggregate portion of the mixture results in mortar rather than concrete. If the proportion of water is too large, the resultant mortar is weak and crumbly.
Other problems occur when such motor is placed in a sealed container, as is often the case in connection with solidified waste burial. Excess water that does not become part of the hydration of the Portland cement is left as standing or free water, which easily may contain radioactivity in a non-solidified form. This free water also acts as a corrosive agent on the metal drum thereby increasing the chance of leaks.
Another disadvantage of Portland cement arises from its high specific gravity and the normal tendency for the particles of Portland cement to sink through the water to the bottom of the container and provide non-uniform distribution in the mixture. Even though the mixing is done correctly, and the wide range of pH values is compensated for in some manner so the cement will set, the solidification of the mass requires an inordinate time before it can be transported and buried. The liquid radioactive waste disposal system of the present invention overcomes most of the disadvantages of the previous systems, and makes possible a more effective way of solidifying and disposing of liquid and solid radioactive waste materials than has heretofore been available. To this end, the present system utilizes a setting agent capable of solidifying large quantities of water (the liquid most often encountered in radioactive waste disposal problems) and holding this water and other radioactive waste materials in a condition particularly suited for transportation and burial.
The described setting agent is a water extendable polymer consisting of an aqueous suspension of urea formaldehyde, usually in partially polymerized form. This material is very "forgiving" in the critical areas of surface tension and pH, and can be used in many proportions to form solids of various strengths. The agent is capable of taking up comparatively large volumes of water as the mass solidifies. Control over the rate of solidification is easily obtained by varying the amount of curing agent used, and the curing agent is a low cost commercially available reducing agent. Increasing the concentration of the curing agent shortens the solidification time as well as the time necessary to obtain the full potential strength of the mass. Raising the temperature of the mass also speeds up the solidification action.
The present system includes a plurality of positive displacement pumps whose output is proportional to their speed of operation, manifolding and valve systems for effecting desired mixtures and proportions, mixers for achieving and maintaining desired interdispersion of the materials, liquid presence and radioactivity sensors, and controls making it possible to operate the entire system from remote locations. The construction of the various elements of the system affords numerous safeguards and safety interlocks against accidental spilling or other escape of radioactive materials and for reducing adverse consequences from any such accidental spills.
The proportioning aspects of the system make it possible to provide a resulting solidified mass which meets LSA (Low Specific Activity) Standards imposed in connection with transportation and handling of radioactive materials. With the levels of radioactivity encountered in nuclear reactor waste materials, it is usually necessary to provide shielding during transportation and handling. This shielding is usually lead, often in the form of lead casks, and of course is extremely heavy. Since the cost of transportation of radioactive waste is more significant than either the cost of solidification or burial costs, any reduction in the amount of shielding required for a particular quantity of radioactive waste material represents a real saving. The present system makes it possible to control the LSA levels in such manner as to provide a reduction in the amount of transport shielding required, resulting in significant monetary savings as well as reducing the risk of radioactive contamination in the event of an accident. The system also provides a mode in which solid materials having a higher radioactivity level may be concentrated in the central area of the solidified mass, so the surrounding, lower radioactivity portions of the mass in themselves act as a shield.
It is therefore a principal object of the present invention to provide a system capable of solidifying liquid radioactive waste into a free standing hardened mass suitable for disposal by burial in the earth or at sea.
Another object of the invention is to provide a system of the character described which is capable of controlling the physical attributes of the resulting solidified mass, and which is especially adapted for inclusion in a surrounding container.
A further object of the present invention is to provide a system of the character described which is capable of controlling the average specific radioactivity throughout the solidified mass, and in which the overall LSA count can be held within prescribed limits. Another object of the invention is to provide a system of the character set forth which is capable of diverting relatively high radioactivity waste material to the central area of the resulting solidified mass, whereby the surrounding lower radioactivity portions may act as a shield.
A still further object of the present invention is to provide a system of the character described which is capable of producing a solidified, hardened mass of liquid radioactive waste of a desired LSA count.
Another object of the invention is to provide a system of the character set forth which is capable of intermixing liquid and non-liquid radioactive waste materials with a setting agent in liquid form and a curing agent in liquid form, with the mixing of the setting agent and curing agent taking place upon entry of the liquified mixture into a receiving tank.
Yet another object of the invention is to provide a method of the character described wherein additional amounts of the setting agent and curing agent are mixed together and added to the receiving tank after it has been filled and the contents solidified so as to fill up unoccupied areas of the container and take up free water which may be present.
A still further object of the invention is to provide a method of the character described in which a controlled quantity of an inhibiting agent is added to the liquid mixture to adjust the solidification time. A further object of the invention is to provide a method of the character set forth in which the proportional quantities of the various components are under continual adjustment during the mixing step, and the proportioning is responsive to the physical characteristics of the components entering the mixture.
Another object of the present invention is to provide a system wherein the positive displacement pumps are capable of handling a wide range and variety of liquid, solid and slurrylike radioactive waste materials.
Yet another object of the invention is to provide a system of the character described in which first contact of the setting agent with the curing agent takes place immediately prior to introduction of the mixture into a receiving and solidifying tank, and the system is adapted for flushing with water at the end of the filling cycle. A further object of the invention is to provide a system of the character described which is particularly adapted for operation from remote positions and includes positive safeguards against accidental spills and deleterious consequences of escaping radioactive waste material.
Other objects and features of advantage will become apparent as the specification and claims continue.